The purpose of a catalytic converter is to convert pollutant materials in engine or turbine exhaust, e.g., carbon monoxide, unburned hydrocarbons, nitrogen oxides, etc., to carbon dioxide, nitrogen and water. Conventional catalytic converters utilize a ceramic honeycomb monolith having square straight through openings or cells, catalyst coated alumina beads, or a corrugated thin metal foil honeycomb monolith having a catalyst carried on or supported by the surface. The catalyst is normally a noble metal, e.g., platinum, palladium, rhodium or ruthenium, or a mixture of two or more of such noble metals. The catalyst catalyzes a chemical reaction, oxidation, reduction, or both, whereby the pollutant is converted to a harmless by-product which then passes through the exhaust system to the atmosphere. However, this conversion is not efficient initially when the exhaust gases and converter are relatively cold. To be effective at a high conversion rate, the catalyst and the surface of the converter with which the exhaust gases come in contact must be at a minimum elevated temperature, e.g., 390 F. for carbon monoxide, 570 F. for volatile organic compounds (VOC), and 1000 F. for methane or natural gas. Otherwise, conversion to harmless by-products is poor and cold start pollution of the atmosphere is high. Once the exhaust system has come to its operating temperature, the catalytic converter is optimally effective. Hence, it is necessary to contact relatively cold exhaust gases with hot catalyst to effect satisfactory conversion at engine start-up. Both compression ignited (diesel) and spark ignited engines have this need.
To achieve heating of the catalyst, there is provided an electrically heatable catalytic converter formed of a corrugated thin metal foil monolith which is connected to a voltage source, e.g., a 12 volt or 24 volt automotive battery, and power applied, preferably before and during start-up, to elevate and maintain the temperature of the catalyst at at least about 650 F. Reference may be had to copending application Ser. No. 587,219 filed Sep. 24, 1990 the disclosure of which application is incorporated herein by reference, for details of an electrically heatable catalytic converter core and a power system for effective heating of the metal monolith.
A problem exists with spirally wound or S-wound corrugated thin metal foil catalytic converters which the present invention solves. The corrugated thin metal monolith core is subject to telescoping in a severe test which the devices must pass. Such telescoping can result in destruction of the electrically heatable catalytic converter. This test involves oscillating (100-200 Hertz and 28 to 60 G) the device in a vertical attitude at high temperature (between 700 and 950 C.) with exhaust gas from a running internal combustion engine being passed through the device. If the wound core device telescopes in the direction of gas flow or breaks up after a predetermined time e.g., 5-200 hours, the device is said to fail the test. Usually, the device will fail in 5 hours if it is going to fail.
It is a principal object of the present invention to provide a device which will pass the foregoing test, and thereby be reliable in extreme service conditions..
Reference may be had to U.S. Pat. No. 3,768,982 to Kitzner dated Oct. 30, 1973. In this patent, heat from a centrally located electric heater is transferred by conduction through a monolithic catalyst support to heat the catalyst to optimum operating temperature. Reference may also be had to U.S. Pat. No. 3,770,389 to Kitzner dated Oct. 30, 1973 which discloses a central electrically heated core within a ceramic monolith, heat being transmitted by conduction to the catalyst contained in the openings of the ceramic monolith. The heating core is formed of metal sheets, one corrugated and the other flat, coated with alumina and also bearing a catalyst. The metallic core is heated electrically by virtue of its own electrical resistance. Heating by conduction takes too long to solve the problem of of atmospheric pollution at start-up. Moreover, the thin metal cores of the present invention do not require a flat thin metal sheet in alternating relation with a corrugated thin metal sheet. A flat thin metal sheet adds about 20% to 30% more weight to the device and necessitates a longer heat-up time or higher power input.
Reference may also be had to U.S. Pat. No. 4,711,009 to Cornelison et al dated Dec. 8, 1987 for details of a process for the preparation of corrugated thin metal foil with a refractory metal oxide coating thereon and having a noble metal catalyst deposited on the oxide surface. This foil may be accordion folded or spirally wound to form the electrically heatable monoliths hereof. This patent is incorporated herein by reference thereto.
Reference may also be had to International PCT publication number WO 89/10471 filed Nov. 2, 1989 which discloses an electrically conductive honeycomb catalyst support unit useful in automobiles. To obtain a suitable electrical resistance between 0.03 and 2 ohms, the honeycomb body is subdivided electrically, cross-sectionally and or axially, by gaps and/or electrically insulating intermediate layers or coatings so that at least one electrical current path having the desired resistance is obtained. Heating is controlled by a timed relay. Separate catalytic converters in the exhaust line, one or more electrically heatable, the other conventional, are disclosed. The basic devices shown in this application and its companion WO 89/10470 filed Nov. 2, 1989 may be used in the present invention. Spiral wound or S-wound cores are subject to the same problem of telescoping under the severe test above described.
In the following description, reference will be made to "ferritic" stainless steel. A suitable formulation for this material is described in U.S. Pat. No. 4,414,023 dated Nov. 8, 1983 to Aggens et al. A specific ferritic stainless steel useful herein contains 20% chromium, 5% aluminum, and from 0.002% to 0.05% of at least one rare earth metal selected from cerium, lanthanum, neodymium, yttrium, and praseodymium, balance iron and steel making impurities.
In the following description, reference will also be made to fibrous ceramic mat or felt. Reference may be had to U.S. Pat. No. 3,795,524 dated Mar. 5, 1974 to Bowman for formulations and manufacture of ceramic fibers useful herein. One such material is currently commercially available under the registered trademark "INTERAM" from 3-M.
Reference will also be had to certain pending applications, i.e., Ser. No. 524,284 filed Apr. 16, 1990, and Ser. No. 587,219. These applications are commonly owned with the present application, and the disclosures thereof are incorporated herein by reference thereto.